dt-bindings: power: supply: qcom,pmi8998: Add missing power-supply ref

Reference the common power supply schema to bring the definition of
monitored-battery property.  Drop also the obvious description because
it is duplicating what is in power-supply.yaml.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20250604-dt-bindings-psy-monitored-battery-v1-3-7f755ff75218@linaro.org
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>

Author: krzk

Documentation/devicetree/bindings/power/supply/qcom,pmi8998-charger.yaml

@@ -38,9 +38,7 @@ properties:
       - const: usbin_i
       - const: usbin_v
 
-  monitored-battery:
-    description: phandle to the simple-battery node
-    $ref: /schemas/types.yaml#/definitions/phandle
+  monitored-battery: true
 
 required:
   - compatible
@@ -51,6 +49,9 @@ required:
   - io-channel-names
   - monitored-battery
 
+allOf:
+  - $ref: power-supply.yaml#
+
 additionalProperties: false
 
 examples: